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Puneet Gupta
Puneet Gupta

Posted on • Originally published at pg-blogs.netlify.app

The Model Context Protocol in Java

#ai #java #llm #mcp

Introduction

Every agent needs tools, and every tool needs a way to reach the model. Building Agentic Workflows in Java built that connection by hand — a hand-written Tool schema, a loop that dispatches on toolUse.name(). LLM Frameworks vs. the Raw SDK in Java showed LangChain4j and Spring AI turning an annotated Java method into that same schema via reflection. Both are still bespoke: the tool lives inside one process, wired to one agent, in one language.

The Model Context Protocol (MCP) solves a different problem: it standardizes the wire format between an AI application and a tool server, so the server doesn't have to be rewritten per agent, per framework, or per language. This post covers what that buys you, builds a minimal MCP server and a client that consumes it — both on the official Java SDK — and gives an honest answer to when reaching for a protocol is worth it over a direct tool call.

The Problem MCP Solves

Without a shared protocol, every pairing of agent framework and tool needs its own glue code: a LangChain4j tool wrapper, a Spring AI @Tool method, a hand-rolled schema for the raw SDK — three integrations for one capability, repeated for every tool and every framework you add. That's an M×N integration problem.

MCP flattens it to M+N. A server exposes tools, resources, and prompts once, over a standard JSON-RPC protocol. Any host application — Claude Code, Claude Desktop, VS Code, or your own agent — creates an MCP client that speaks that same protocol, regardless of which framework built the host. Write the server once; every MCP-aware host can use it without new integration code.

The protocol itself is intentionally boring: JSON-RPC 2.0 messages for lifecycle negotiation, tool discovery, and tool execution. Discovery (tools/list) and execution (tools/call) are the two calls that matter for this post:

// tools/list response (abbreviated)
{ "jsonrpc": "2.0", "id": 2, "result": { "tools": [
  { "name": "get_account_balance", "description": "Look up the balance for an account by ID",
    "inputSchema": { "type": "object",
      "properties": { "accountId": { "type": "string" } }, "required": ["accountId"] } }
]}}
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Two transports cover most cases: stdio (a local subprocess, one client per server — what this post uses) and Streamable HTTP (a remote server, many clients, standard bearer-token auth). Source: MCP architecture overview.

Building a Minimal MCP Server

The official Java SDK (io.modelcontextprotocol.sdk, Tier 2) needs no external framework — its core mcp module ships stdio and HTTP transports directly (Java MCP Server docs):

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>io.modelcontextprotocol.sdk</groupId>
      <artifactId>mcp-bom</artifactId>
      <version>2.0.0</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>
<dependencies>
  <dependency>
    <groupId>io.modelcontextprotocol.sdk</groupId>
    <artifactId>mcp</artifactId>
  </dependency>
</dependencies>
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The tool is a read-only account-balance lookup — deliberately simple, and deliberately keyed by ID rather than a free-text query, for reasons the next section makes concrete:

import io.modelcontextprotocol.server.McpServer;
import io.modelcontextprotocol.server.McpSyncServer;
import io.modelcontextprotocol.server.McpServerFeatures.SyncToolSpecification;
import io.modelcontextprotocol.server.transport.StdioServerTransportProvider;
import io.modelcontextprotocol.json.McpJsonDefaults;
import io.modelcontextprotocol.spec.McpSchema.*;
import java.util.List;
import java.util.Map;
import java.util.regex.Pattern;

// A fixed, in-memory dataset — stands in for a real accounts service
private static final Map<String, Double> ACCOUNTS = Map.of(
    "ACC-100001", 542.10, "ACC-100002", 12.50);
private static final Pattern ACCOUNT_ID = Pattern.compile("^ACC-\\d{6}$");

Map<String, Object> inputSchema = Map.of(
    "type", "object",
    "properties", Map.of("accountId", Map.of("type", "string",
        "description", "Account ID, e.g. ACC-100001")),
    "required", List.of("accountId"));

SyncToolSpecification balanceTool = SyncToolSpecification.builder()
    .tool(Tool.builder("get_account_balance", inputSchema)
        .description("Look up the balance for an account by ID")
        .build())
    .callHandler((exchange, request) -> {
        Object raw = request.arguments().get("accountId");
        // Validate/whitelist BEFORE using — request.arguments() is model-provided, untrusted data
        if (!(raw instanceof String accountId) || !ACCOUNT_ID.matcher(accountId).matches()) {
            return CallToolResult.builder()
                .content(List.of(TextContent.builder("Invalid account ID format").build()))
                .isError(true).build();
        }
        Double balance = ACCOUNTS.get(accountId);
        String text = balance != null ? "Balance for " + accountId + ": $" + balance
                                       : "No account found for " + accountId;
        return CallToolResult.builder().content(List.of(TextContent.builder(text).build())).build();
    })
    .build();

StdioServerTransportProvider transportProvider =
    new StdioServerTransportProvider(McpJsonDefaults.getMapper());

McpSyncServer server = McpServer.sync(transportProvider)
    .serverInfo("accounts-server", "1.0.0")
    .capabilities(ServerCapabilities.builder().tools(true).build())
    .build();

server.addTool(balanceTool);
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A read-only resource — MCP's second primitive, for data the model can load into context without an action attached — is a few lines more:

server.addResource(new McpServerFeatures.SyncResourceSpecification(
    Resource.builder("docs://refund-policy", "Refund Policy")
        .description("Current refund policy text").mimeType("text/plain").build(),
    (exchange, request) -> ReadResourceResult.builder(List.of(TextResourceContents.builder(
        "docs://refund-policy", "Refunds are issued within 5 business days.").build())).build()));
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Source for the tool/resource specification shapes: Java MCP Server docs. StdioServerTransportProvider communicates over stdin/stdout — never System.out.println in a stdio server; it corrupts the JSON-RPC stream (the same logging warning every language's quickstart repeats).

The Trust Boundary Doesn't Move

request.arguments() in a tool's call handler is the same untrusted, model-provided data as toolUse.input() in the raw SDK loop (post 14) or the arguments a @Tool-annotated method receives (post 26) — MCP standardizes the wire format, not the trust model. The SDK does validate the shape of arguments against inputSchema before your handler ever runs (a JSON Schema 2020-12 check, on by default, per the migration notes) — but shape validation only confirms accountId is a string, not that it's a safe string. The regex check above still has to happen inside the handler.

Never skip that second check by trusting the schema alone:

// UNSAFE — never do this: string-interpolating a tool argument into a query
String sql = "SELECT balance FROM accounts WHERE id = '" + accountId + "'"; // SQL injection

// SAFE — validate/whitelist first (as balanceTool does above), then use a fixed lookup
// or a parameterized query — never string-built from model input
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The lesson from post 14's executeValidatedTool carries over unchanged: whitelist the shape you expect, reject anything else, and never let a tool argument reach a shell command, file path, or SQL string by concatenation.

Building a Client That Consumes the Server

A client connects over the same stdio transport, discovers the tool, and calls it — no knowledge of how the server was implemented:

import io.modelcontextprotocol.client.McpClient;
import io.modelcontextprotocol.client.McpSyncClient;
import io.modelcontextprotocol.client.transport.ServerParameters;
import io.modelcontextprotocol.client.transport.StdioClientTransport;
import io.modelcontextprotocol.spec.McpSchema.*;

ServerParameters params = ServerParameters.builder("java")
    .args("-jar", "/path/to/accounts-mcp-server.jar")
    .build();
var transport = new StdioClientTransport(params, McpJsonDefaults.getMapper());

McpSyncClient client = McpClient.sync(transport).build();
client.initialize();

ListToolsResult tools = client.listTools();
System.out.println("Available: " + tools.tools().stream().map(Tool::name).toList());

CallToolResult result = client.callTool(
    CallToolRequest.builder("get_account_balance")
        .arguments(Map.of("accountId", "ACC-100001"))
        .build());

client.closeGracefully();
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Source for McpClient/ServerParameters/StdioClientTransport: Java MCP Client docs. This is the same shape modelcontextprotocol.io's own Build an MCP Server tutorial uses to test a Spring AI weather server from a plain Java client — the client doesn't care which SDK or framework built the server on the other end of the transport.

Wiring MCP Into the Agent Loop from Post 14

The agent loop itself doesn't change — only where the tool list and tool execution come from. Instead of hand-written Tool objects and a local executeValidatedTool method, list tools from the MCP client and route tool_use blocks through client.callTool(...):

import com.anthropic.core.JsonValue;

// Convert an MCP Tool's Map-shaped inputSchema into the Anthropic SDK's Tool.InputSchema
// (same InputSchema.builder() shape post 26 hand-writes for getWeatherTool/calculateTool)
@SuppressWarnings("unchecked")
private static com.anthropic.models.messages.Tool toAnthropicTool(Tool mcpTool) {
    Map<String, Object> props = (Map<String, Object>) mcpTool.inputSchema().getOrDefault("properties", Map.of());
    List<String> required = (List<String>) mcpTool.inputSchema().getOrDefault("required", List.of());
    var propsBuilder = com.anthropic.models.messages.Tool.InputSchema.Properties.builder();
    props.forEach((name, schema) -> propsBuilder.putAdditionalProperty(name, JsonValue.from(schema)));
    return com.anthropic.models.messages.Tool.builder()
        .name(mcpTool.name()).description(mcpTool.description())
        .inputSchema(com.anthropic.models.messages.Tool.InputSchema.builder()
            .properties(propsBuilder.build()).required(required).build())
        .build();
}

ListToolsResult mcpTools = client.listTools();
List<com.anthropic.models.messages.Tool> anthropicTools =
    mcpTools.tools().stream().map(ToolBridge::toAnthropicTool).toList();

// Inside the loop from post 14, in place of executeValidatedTool(toolUse.name(), toolUse.input()):
CallToolResult mcpResult = client.callTool(
    CallToolRequest.builder(toolUse.name()).arguments(toolUse.input()).build());
String textResult = mcpResult.content().stream()
    .filter(TextContent.class::isInstance).map(c -> ((TextContent) c).text())
    .findFirst().orElse("");
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Everything else from post 14 — the MAX_ITERATIONS cap, claude-opus-4-8 with ThinkingConfigAdaptive, the message bookkeeping — is unchanged. Only the tool's implementation moved to a separate, reusable process.

MCP vs. the Framework Tool Abstraction (Post 26)

LangChain4j's @Tool/AiServices and Spring AI's @Tool/ChatClient (post 26) wrap a method living in your own JVM process — convenient, but the tool only exists inside that one application. MCP wraps a separate process or service behind a standard protocol, so the same accounts server above can be launched by Claude Desktop, called from Claude Code, and consumed by this post's plain-Java client — three unrelated hosts, zero rewritten integration code (the "broad ecosystem support" modelcontextprotocol.io advertises).

The two aren't mutually exclusive: Spring AI ships its own MCP client boot starter, so a Spring Boot agent can consume this exact server as auto-configured tools instead of hand-rolling the McpClient above:

<dependency>
    <groupId>org.springframework.ai</groupId>
    <artifactId>spring-ai-starter-mcp-client</artifactId>
</dependency>
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with spring.ai.mcp.client.stdio.servers-configuration pointing at a server-launch config — confirmed in modelcontextprotocol.io's own Build an MCP Server Java walkthrough. Framework tool abstractions and MCP solve different layers of the same problem: one wires a method into a prompt; the other lets that method's server outlive any one framework.

When MCP Earns Its Weight — and When It's Overkill

Reach for MCP when the tool or data source needs to be shared across independently-built AI applications — your accounts lookup used by Claude Desktop for support staff and Claude Code for engineers and a custom internal agent — or when you're consuming someone else's server (Sentry, GitHub, a filesystem) without writing integration code per host. It's the right call when the tool is a product in its own right, not a private implementation detail of one agent.

It's ceremony when the tool has exactly one caller in one codebase: a separate process, a stdio or HTTP transport, and a protocol handshake are pure overhead next to a @Tool-annotated method (post 26) or a hand-written Tool schema (post 14) that never leaves the JVM. Build the direct version first; promote it to an MCP server only once a second, independent host actually needs to call it.

Practical Checklist

Practice Why it matters
Start with a direct tool call (post 14/26); promote to MCP only when a second host needs it MCP's cost — a process, a transport, a protocol — buys nothing for a single caller
Validate/whitelist request.arguments() inside the handler, even with schema validation on Schema validation checks shape, not safety — get_account_balance still checks the regex
Never string-interpolate a tool argument into SQL, a shell command, or a file path The same rule as posts 14/26 — MCP doesn't change the trust boundary
Never write to stdout in a stdio-transport server Corrupts the JSON-RPC stream; log to stderr or a file instead
Prefer a fixed lookup or parameterized query over building a query string from model input The account-balance tool above never builds a string query from accountId
Cite a live source for every MCP class/method before shipping The SDK is Tier 2 and evolving; a stale snippet is worse than none

Final Thoughts

MCP doesn't replace the agentic loop from post 14 or the framework tool abstractions from post 26 — it standardizes what sits behind either one: a tool server that any compliant client can discover and call, without per-host glue code. That standardization is worth real engineering cost when a tool needs to serve more than one application; for a tool with one caller, the direct approaches this series already covered remain the simpler, and correct, default.

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